Tip gap control systems with active blade tips

ABSTRACT

A tip gap control system for a ducted aircraft includes a flight control computer including a blade length control module configured to generate a blade tip actuator command and a proprotor system in data communication with the flight control computer. The proprotor system includes a duct and proprotor blades surrounded by the duct. Each of the proprotor blades includes an active blade tip movable into various positions including a retracted position and an extended position. The tip gap control system also includes one or more actuators coupled to the active blade tips. The one or more actuators move the active blade tips between the various positions based on the blade tip actuator command, thereby controlling a tip gap between the proprotor blades and the duct.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to aircraft having ductedrotor systems and, in particular, to tip gap monitoring and controlsystems that actively, semi-actively or passively manage the tip gapbetween the rotor blades and duct of a ducted rotor system.

BACKGROUND

Ducted rotor systems offer several benefits over open rotor systems inwhich the rotor blades are exposed. For example, ducted rotor systemsemit less noise and are therefore preferred when a reduced noiseenvironment is desired, such as during air reconnaissance, clandestineoperations or flight in urban airspace. Ducts increase safety for groundpersonnel and crew by preventing contact with an operating rotor. Openlyexposed rotors can lead to blade tip thrust losses during flight. Byreducing rotor blade tip losses, a ducted rotor system is more efficientin producing thrust than an open rotor system of similar diameter,especially at low speed and high static thrust levels. Also, the thrustvectoring capabilities of open rotor systems are limited as is the useof pressure differentials to augment thrust.

The performance of a ducted rotor system is sensitive to the tip gapbetween the blade tips and the duct. Ducted rotor systems are designedto have a minimum tip gap throughout the flight envelope to maximizeduct performance. For example, ducted rotor systems are most efficientin hover when the tip gap is as small as possible. Conversely, the tipgap must be large enough to avoid collisions between the rotor bladesand the duct during operation. Furthermore, the tip gap of a ductedrotor system changes during flight as the duct, rotor blades or statorsdeform or deflect under load in response to flight conditions. Forexample, on tiltrotor aircraft utilizing ducted proprotor systems, theducted proprotor systems experience high loads in the transition betweenthe vertical takeoff and landing flight mode and the forward flightmode. These high transition loads can deform the duct, proprotor blades,stators or other parts of the ducted proprotor system, which affects thetip gap. Unexpected collisions with ducted rotor systems during flight,such as bird strikes, also affect the tip gap. Current ducted aircraftare unable to monitor, manage and control the changing tip gap of theirducted rotors, which necessitates shortening the rotor blades more thannecessary to prevent collisions between the rotor blades and duct, whichin turn leads to performance degradation. Accordingly, a need has arisenfor tip gap monitoring and control systems that actively, semi-activelyor passively enable the rotor blades of a ducted rotor system to be asclose as possible to the duct while mitigating the risk of collisionbetween the rotor blades and the duct.

SUMMARY

In a first aspect, the present disclosure is directed to a tip gapcontrol system for a ducted aircraft. The tip gap control systemincludes a flight control computer including a blade length controlmodule configured to generate a blade tip actuator command and aproprotor system in data communication with the flight control computer.The proprotor system includes a duct and proprotor blades surrounded bythe duct. Each of the proprotor blades includes an active blade tipmovable into various positions including a retracted position and anextended position. The tip gap control system also includes one or moreactuators coupled to the active blade tips. The one or more actuatorsmove the active blade tips between the various positions based on theblade tip actuator command, thereby controlling a tip gap between theproprotor blades and the duct.

In some embodiments, the blade tip actuator command may include a tipgap adjustment distance, the one or more actuators configured to movethe active blade tips by the tip gap adjustment distance. In certainembodiments, the blade tip actuator command may include one or moreblade-specific blade tip actuator commands, each of the blade-specificblade tip actuator commands corresponding to a respective one of theactive blade tips. In some embodiments, the blade tip actuator commandmay include one of a retract or an extend command, the one or moreactuators configured to move the active blade tips by a predetermineddistance in response to receiving the blade tip actuator command. Incertain embodiments, each of the proprotor blades may include a mainbody having an open distal end, the active blade tips slidably coupledto the open distal ends of the main bodies of the proprotor blades. Insuch embodiments, the active blade tips may be at least partiallyretracted into the main bodies of the proprotor blades in the retractedposition and at least partially extended from the open distal ends ofthe main bodies of the proprotor blades in the extended position,thereby increasing the tip gap in the retracted position.

In some embodiments, each of the proprotor blades may include a mainbody having a distal end, the active blade tips hingeably coupled to thedistal ends of the main bodies of the proprotor blades. In suchembodiments, the active blade tips may be substantially coplanar withthe main bodies of the proprotor blades in the extended position andform an angle of less than 180 degrees with the main bodies of theproprotor blades in the retracted position, thereby increasing the tipgap in the retracted position. In certain embodiments, each proprotorblade may include a spring biasing a respective active blade tip in theretracted position or the extended position. In some embodiments, theactive blade tips may be independently actuated to permit nonuniformpositioning of the active blade tips. In certain embodiments, theproprotor system may include a proprotor hub, the one or more actuatorsdisposed at the proprotor hub. In such embodiments, the one or moreactuators may be coupled to the active blade tips by a plurality ofspanwise links. In some embodiments, one or more actuators may bedisposed at distal ends of the proprotor blades, each actuator coupledto a respective one of the active blade tips.

In a second aspect, the present disclosure is directed to a rotorcraftincluding a fuselage, a flight control computer including a blade lengthcontrol module configured to generate a blade tip actuator command and aproprotor system coupled to the fuselage and in data communication withthe flight control computer. The proprotor system includes a duct andproprotor blades surrounded by the duct. Each of the proprotor bladesincludes an active blade tip movable into various positions including aretracted position and an extended position. The proprotor system alsoincludes one or more actuators coupled to the active blade tips. The oneor more actuators move the active blade tips between various positionsbased on the blade tip actuator command, thereby controlling a tip gapbetween the proprotor blades and the duct.

In some embodiments, the rotorcraft may include a maneuver detectionmodule configured to detect a flight condition of the rotorcraft such asa flight maneuver or a flight mode, the blade length control moduleconfigured to determine the blade tip actuator command based on theflight condition.

In a third aspect, the present disclosure is directed to a method forcontrolling a tip gap for a ducted aircraft including generating a bladetip actuator command; transmitting the blade tip actuator command to aproprotor system including a duct and proprotor blades, the proprotorblades including active blade tips; and moving at least one of theactive blade tips between a retracted position and an extended positionin response to the blade tip actuator command, thereby controlling thetip gap between the proprotor blades and the duct.

In some embodiments, the method may include generating the blade tipactuator command in response to receiving a tip gap adjustment distance.In certain embodiments, the method may include generating the blade tipactuator command in response to the tip gap adjustment distanceexceeding a tip gap adjustment distance threshold. In some embodiments,the method may include generating the blade tip actuator command basedon a pitch of the proprotor blades. In certain embodiments, the ductedaircraft may be convertible between a vertical takeoff and landingflight mode and a forward flight mode. In such embodiments, the methodmay include moving the active blade tips between the retracted positionand the extended position in response to the ducted aircraft convertingbetween the vertical takeoff and landing flight mode and the forwardflight mode. In some embodiments, the method may include retracting theactive blade tips in response to detecting a structural deformity of theproprotor system. In certain embodiments, the method may includeretracting the active blade tips in response to detecting a collisionwith the proprotor system. In some embodiments, the method may includemoving the active blade tips such that the tip gap is substantiallyequal to a tip gap target.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1F are schematic illustrations of a ducted aircraft having atip gap monitoring and control system in accordance with embodiments ofthe present disclosure;

FIG. 2 is a block diagram of a propulsion and control system for aducted aircraft having a tip gap monitoring and control system inaccordance with embodiments of the present disclosure;

FIG. 3 is a block diagram of a control system for a ducted aircrafthaving a tip gap monitoring and control system in accordance withembodiments of the present disclosure;

FIG. 4 is a schematic illustration of a tip gap monitoring and controlsystem for a ducted aircraft in accordance with embodiments of thepresent disclosure;

FIGS. 5A-5B are front views of a tip gap monitoring system utilizingdistance sensors in accordance with embodiments of the presentdisclosure;

FIGS. 6A-6G are various views of a tip gap control system utilizingactive blade tips in accordance with embodiments of the presentdisclosure;

FIGS. 7A-7G are various views of a tip gap control system utilizingactive inner duct surfaces in accordance with embodiments of the presentdisclosure;

FIGS. 8A-8H are schematic illustrations of a ducted aircraft having atip gap monitoring and control system in a sequential flight operatingscenario in accordance with embodiments of the present disclosure;

FIGS. 9A-9C are flowcharts of various methods for monitoring andcontrolling the tip gap for a ducted aircraft in accordance withembodiments of the present disclosure;

FIGS. 10A-10E are various views of a passive tip gap control system fora ducted aircraft utilizing blade tip extensions in accordance withembodiments of the present disclosure;

FIGS. 11A-11H are schematic illustrations of a ducted aircraft having apassive tip gap control system in a sequential flight operating scenarioin accordance with embodiments of the present disclosure;

FIGS. 12A-12F are various views of a passive tip gap control system fora ducted aircraft utilizing tension-torsion straps in accordance withembodiments of the present disclosure;

FIGS. 13A-13E are various views of a proprotor system having sacrificialblade tips in accordance with embodiments of the present disclosure; and

FIGS. 14A-14D are various views of a proprotor system having sacrificialblade tips including a frangible lattice structure in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,all features of an actual implementation may not be described in thisspecification. It will of course be appreciated that in the developmentof any such actual embodiment, numerous implementation-specificdecisions must be made to achieve the developer's specific goals, suchas compliance with system-related and business-related constraints,which will vary from one implementation to another. Moreover, it will beappreciated that such a development effort might be complex andtime-consuming but would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicesdescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including by mere contact or by moving and/or non-movingmechanical connections.

Referring to FIGS. 1A-1F in the drawings, various views of a ductedaircraft 10 having a tip gap monitoring and control system are depicted.FIGS. 1A, 1C and 1E depict ducted aircraft 10 in a vertical takeoff andlanding (VTOL) orientation wherein the proprotor systems providethrust-borne lift. FIGS. 1B, 1D and 1F depict ducted aircraft 10 in aforward flight orientation wherein the proprotor systems provide forwardthrust with the forward airspeed of ducted aircraft 10 providingwing-borne lift, thereby enabling ducted aircraft 10 to have a highspeed and/or high endurance forward flight mode. Ducted aircraft 10 hasa longitudinal axis 10 a that may also be referred to as the roll axis,a lateral axis 10 b that may also be referred to as the pitch axis and avertical axis 10 c that may also be referred to as the yaw axis, as bestseen in FIGS. 1A-1B. As illustrated, when longitudinal axis 10 a andlateral axis 10 b are both in a horizontal plane that is normal to thelocal vertical in the earth's reference frame, ducted aircraft 10 has alevel flight attitude.

In the illustrated embodiment, ducted aircraft 10 has an airframe 12including a fuselage 14, wings 16 a, 16 b and a tail assembly 18. Wings16 a, 16 b have an airfoil cross-section that generates lift responsiveto the forward airspeed of ducted aircraft 10. In the illustratedembodiment, wings 16 a, 16 b are straight wings with a tapered leadingedge. It will be appreciated, however, that wings 16 a, 16 b may be of awide variety of shapes, sizes and configurations, depending upon theperformance characteristics desired. In the illustrated embodiment,wings 16 a, 16 b include ailerons to aid in roll and/or pitch control ofducted aircraft 10 during forward flight. Tail assembly 18 is depictedas a vertical fin, or stabilizer, that may include one or more ruddersto control the yaw of ducted aircraft 10 during forward flight. In otherembodiments, tail assembly 18 may have two or more vertical fins and/ora horizontal stabilizer that may include one or more elevators tocontrol the pitch of ducted aircraft 10 during forward flight. It willbe appreciated, however, that tail assembly 18 may be of a wide varietyof shapes, sizes and configurations, depending upon the performancecharacteristics desired.

In the illustrated embodiment, ducted aircraft 10 includes fourproprotor systems forming a two-dimensional distributed thrust arraythat is coupled to airframe 12. As used herein, the term“two-dimensional thrust array” refers to a plurality of thrustgenerating elements that occupy a two-dimensional space in the form of aplane. As used herein, the term “distributed thrust array” refers to theuse of multiple thrust generating elements, each producing a portion ofthe total thrust output. The thrust array of ducted aircraft 10 includesa forward-port proprotor system 20 a, a forward-starboard proprotorsystem 20 b, an aft-port proprotor system 20 c and an aft-starboardproprotor system 20 d, which may be referred to collectively asproprotor systems 20. Forward-port proprotor system 20 a andforward-starboard proprotor system 20 b are each rotatably mounted to ashoulder portion of fuselage 12 at a forward station thereof. Aft-portproprotor system 20 c is rotatably mounted on the outboard end of wing16 a. Aft-starboard proprotor system 20 d is rotatably mounted on theoutboard end of wing 16 b. Proprotor systems 20 may each include atleast one variable speed electric motor and a speed controllerconfigured to provide variable speed control to the proprotor assemblyover a wide range of rotor speeds.

When ducted aircraft 10 is operating in the VTOL flight mode andsupported by thrust-borne lift, proprotor systems 20 each have agenerally horizontal position such that the proprotor assemblies arerotating in generally the same horizontal plane, as best seen in FIGS.1C and 1E. When ducted aircraft 10 is operating in the forward flightmode and supported by wing-borne lift, proprotor systems 20 each have agenerally vertical position with the forward proprotor assembliesrotating generally in a forward vertical plane and the aft proprotorassemblies rotating generally in an aft vertical plane, as best seen inFIG. 1F. Transitions between the VTOL flight mode and the forward flightmode of ducted aircraft 10 are achieved by changing the angularpositions of proprotor systems 20 between their generally horizontalpositions and their generally vertical positions as discussed herein.

Ducted aircraft 10 may include a liquid fuel powered turbo-generatorthat includes a gas turbine engine and an electric generator.Preferably, the electric generator charges an array of batteries thatprovides power to the electric motors of proprotor systems 20 via apower management system. In other embodiments, the turbo-generator mayprovide power directly to the power management system and/or theelectric motors of proprotor systems 20. In yet other embodiments,proprotor systems 20 may be mechanically driven by the power plant ofducted aircraft 10 via suitable gearing, shafting and clutching systems.

Ducted aircraft 10 has a fly-by-wire control system that includes aflight control computer 22 that is preferably a redundant digital flightcontrol system including multiple independent flight control computers.Flight control computer 22 preferably includes non-transitory computerreadable storage media including a set of computer instructionsexecutable by one or more processors for controlling the operation ofducted aircraft 10. Flight control computer 22 may be implemented on oneor more general-purpose computers, special purpose computers or othermachines with memory and processing capability. Flight control computer22 may include one or more memory storage modules including randomaccess memory, non-volatile memory, removable memory or other suitablememory. Flight control computer 22 may be a microprocessor-based systemoperable to execute program code in the form of machine-executableinstructions. Flight control computer 22 may be connected to othercomputer systems via a suitable communications network that may includeboth wired and wireless connections.

Flight control computer 22 communicates via a wired communicationsnetwork within airframe 12 with the electronics nodes of each proprotorsystem 20. Flight control computer 22 receives sensor data from andsends flight command information to proprotor systems 20 such that eachproprotor system 20 may be individually and independently controlled andoperated. For example, flight control computer 22 is operable toindividually and independently control the proprotor speed andcollective blade pitch of each proprotor system 20 as well as theangular position of each proprotor system 20. Flight control computer 22may autonomously control some or all aspects of flight operation forducted aircraft 10. Flight control computer 22 is also operable tocommunicate with remote systems, such as a ground station via a wirelesscommunications protocol. The remote system may be operable to receiveflight data from and provide commands to flight control computer 22 toenable remote flight control over some or all aspects of flightoperation for ducted aircraft 10. In addition, ducted aircraft 10 may bepilot operated such that a pilot interacts with a pilot interface thatreceives flight data from and provides commands to flight controlcomputer 22 to enable onboard pilot control over some or all aspects offlight operation for ducted aircraft 10.

Ducted aircraft 10 includes landing gear 24 for ground operations.Landing gear 24 may include passively operated pneumatic landing strutsor actively operated landing struts. In the illustrated embodiment,landing gear 24 includes a plurality of wheels that enable ductedaircraft 10 to taxi and perform other ground maneuvers. Landing gear 24may include a passive brake system, an active brake system such as anelectromechanical braking system and/or a manual brake system tofacilitate parking as required during ground operations and/or passengeringress and egress.

In the illustrated embodiment, proprotor systems 20 are ducted proprotorsystems each having a five bladed proprotor assembly with variable pitchproprotor blades 26 operable for collective pitch control. In otherembodiments, the number of proprotor blades could be either greater thanor less than five and/or the proprotor blades could have a fixed pitch.Proprotor blades 26 of each proprotor system 20 are surrounded by a duct28, which is supported by stators 30. Duct 28 and stators 30 may beformed from metallic, composite, carbon-based or other sufficientlyrigid materials. The inclusion of duct 28 on each proprotor system 20offers several benefits over open proprotor systems having exposedproprotor blades. For example, proprotor systems 20 emit less noise andare therefore preferred when a reduced noise environment is desired,such as during air reconnaissance, clandestine operations or flight inurban airspace. Ducts 28 increase safety for ground personnel and crewby preventing inadvertent collisions with a spinning proprotor. Openlyexposed proprotors can lead to blade tip thrust losses during flight. Byreducing proprotor blade tip losses, ducted proprotor systems 20 aremore efficient in producing thrust than open proprotor systems ofsimilar diameter, especially at low speed and high static thrust levels.Also, the thrust vectoring capabilities of open rotor systems arelimited as is the use of pressure differentials to augment thrust.

The performance of each proprotor system 20 is sensitive to a tip gap 32between the tips of proprotor blades 26 and the inner surfaces of ducts28. Proprotor systems 20 may be designed to have a minimum tip gap 32throughout the flight envelope to maximize duct performance. Forexample, proprotor systems 20 are most efficient while hovering in theVTOL flight mode when tip gap 32 is as small as possible. Conversely,tip gap 32 must be large enough to avoid collisions between proprotorblades 26 and ducts 28. Tip gap 32 may vary widely depending on the sizeof proprotor systems 20, desired flight attributes and other factors,but by way of non-limiting example is typically in a range between 0.05inches and 1 inch, such as between 0.1 inches and 0.25 inches.

Furthermore, tip gap 32 changes as proprotor blades 26, ducts 28 and/orstators 30 deform or deflect under load and during unexpected events.For example, proprotor systems 20 experience higher loads in theconversion mode between the VTOL flight mode and the forward flightmode, caused in part by the dissymmetry of lift and vibration ofproprotor blades 26 at a sideways angle of attack of proprotor blades 26during the conversion mode. These increased loads can deform proprotorblades 26, ducts 28, stators 30 and/or other parts of proprotor systems20, thereby affecting tip gap 32. Unexpected or imminent collisions withproprotor systems 20 during flight, such as bird strikes, also affecttip gap 32. Current ducted aircraft are unable to monitor and controlthe changing tip gap of their ducted proprotors, which necessitatesshortening their proprotor blades more than necessary to preventcollisions between the proprotor blades and duct, which in turn leads toperformance degradation.

Ducted aircraft 10 includes tip gap monitoring and control systems 34,36 to address these and other tip gap issues of previous aircraft. Tipgap monitoring and control systems 34, 36 enable ducted aircraft 10 toactively or semi-actively enable proprotor blades 26 to be as close aspossible to ducts 28 while mitigating the risk of collision betweenproprotor blades 26 and ducts 28. Tip gap monitoring system 34 includessensors 38 a, 38 b, 38 c coupled to proprotor blades 26, ducts 28 andstators 30, respectively, to detect parameters of proprotor systems 20such as strain, deflection or tip gap distance. These sensormeasurements are transmitted to flight control computer 22 so that tipgap monitoring system 34 can determine tip gap 32 for each proprotorsystem 20 based on the sensor measurements. Tip gap control system 36may then actively or semi-actively adjust tip gap 32 by extending orretracting active blade tips 40 on the distal ends of proprotor blades26 and/or active inner duct surfaces 42 circumferentially disposed onthe inner surface of ducts 28. For example, tip gap control system 36may extend or retract active blade tips 40 and/or active inner ductsurfaces 42 by a tip gap adjustment distance determined by tip gapmonitoring system 34. In other embodiments, tip gap control system 36may control tip gap 32 using active blade tips 40 or active inner ductsurfaces 42 based on the flight mode, flight condition or flightmaneuver of ducted aircraft 10 or other parameters such as the bladepitch of proprotor blades 26. In yet other embodiments, ducted aircraft10 may include a passive tip gap control system that retracts or extendsproprotor blades 26 of each proprotor system 20 based on the collectivepitch of proprotor blades 26. Alternatively, active blade tips 40 may besacrificial blade tips that deform upon contact with ducts 28 to reducedamage to the main bodies of proprotor blades 26 as well as ducts 28.

It should be appreciated that ducted aircraft 10 is merely illustrativeof a variety of aircraft that can implement the embodiments disclosedherein. Indeed, tip gap monitoring and control systems 34, 36, in boththeir active and semi-active implementations, as well as the passive tipgap control systems disclosed herein may be implemented on any aircraftthat utilizes one or more ducts. Other aircraft implementations caninclude hybrid aircraft, tiltwing aircraft, unmanned aircraft,gyrocopters, propeller-driven airplanes, quadcopters, compoundhelicopters, jets, drones and the like. While many of the illustrativeembodiments are described herein as being implemented on ductedproprotors with proprotor blades, the illustrative embodiments may alsobe implemented on rotor blades such as those present on helicopters orquadcopters. Tip gap monitoring and control systems 34, 36 and thepassive tip gap control systems disclosed herein may also be implementedon ducted tail rotors or anti-torque systems. As such, those skilled inthe art will recognize that tip gap monitoring and control systems 34,36 and the passive tip gap control systems disclosed herein can beintegrated into a variety of aircraft configurations. It should beappreciated that even though aircraft are particularly well-suited toimplement the embodiments of the present disclosure, non-aircraftvehicles and devices can also implement the embodiments.

Referring additionally to FIG. 2 in the drawings, various systems ofducted aircraft 10 including tip gap monitoring and control systems 34,36 are depicted. As discussed herein, ducted aircraft 10 includes flightcontrol computer 22 and a two-dimensional distributed thrust arraydepicted as forward-port proprotor system 20 a, forward-starboardproprotor system 20 b, aft-port proprotor system 20 c and aft-starboardproprotor system 20 d. Each proprotor system 20 includes an electronicsnode depicted as having one or more controllers such as an electronicspeed controller, one or more sensors 38 such as strain or distancesensors and one or more actuators 44 such as an active blade tipactuator, an active inner duct surface actuator, a rotor system positionactuator and/or a blade pitch actuator. Each proprotor system 20 alsoincludes at least one variable speed electric motor and a proprotorassembly coupled to the output drive of the electric motor.

Tip gap monitoring system 34 includes sensors 38, which generate sensormeasurements of one or more parameters of proprotor systems 20. Thesensor measurements generated by sensors 38 are transmitted to flightcontrol computer 22, where tip gap monitoring system 34 determines thecurrent tip gap for each proprotor system 20 based on the sensormeasurements. Tip gap monitoring system 34 may then determine tip gapadjustment distances for each proprotor system 20 based on the tip gapscalculated from the sensor measurements. For example, tip gap monitoringsystem 34 may calculate a tip gap adjustment distance by which to adjustthe tip gap of proprotor system 20 a to equal a predetermined tip gaptarget, which may be larger or smaller than the measured tip gap. Tipgap control system 36 may then send actuator commands to actuators 44 ofproprotor systems 20 to extend or retract active blade tips 40 and/oractive inner duct surfaces 42 based on the tip gap adjustmentdistance(s) determined by tip gap monitoring system 34. Moreparticularly, tip gap control system 36 includes a blade length controlmodule 46 to generate and send blade tip actuator commands to actuators44, which move active blade tips 40 of proprotor blades 26. Tip gapcontrol system 36 also includes inner duct surface control module 48 togenerate and send inner duct surface actuator commands to actuators 44to move active inner duct surfaces 42 of ducts 28. The measured tip gapsof proprotor systems 20 may be nonuniform due to the unique loadsexperienced by each proprotor system 20. Therefore, the blade tipactuator commands and inner duct surface actuator commands generated bytip gap control system 36 may likewise be nonuniform for each proprotorsystem 20 such that the tip gaps of proprotor systems 20 may beindependently adjusted.

Referring additionally to FIG. 3 in the drawings, a block diagramdepicts a control system 50 operable for use with ducted aircraft 10 ofthe present disclosure. In the illustrated embodiment, control system 50includes three primary computer based subsystems; namely, an airframesystem 52, a remote system 54 and a pilot system 56. In someimplementations, remote system 54 includes a programming application 58and a remote control application 60. Programming application 58 enablesa user to provide a flight plan and mission information to ductedaircraft 10 such that flight control computer 22 may engage inautonomous control over ducted aircraft 10. For example, programmingapplication 58 may communicate with flight control computer 22 over awired or wireless communication channel 62 to provide a flight planincluding, for example, a starting point, a trail of waypoints and anending point such that flight control computer 22 may use waypointnavigation during the mission.

In the illustrated embodiment, flight control computer 22 is a computerbased system that includes a command module 64 and a monitoring module66. It is to be understood by those skilled in the art that these andother modules executed by flight control computer 22 may be implementedin a variety of forms including hardware, software, firmware, specialpurpose processors and combinations thereof. Flight control computer 22receives input from a variety of sources including internal sources suchas sensors 38, controllers and actuators 44 and proprotor systems 20a-20 d and external sources such as remote system 54 as well as globalpositioning system satellites or other location positioning systems andthe like. During the various operating modes of ducted aircraft 10including the VTOL flight mode, the forward flight mode and transitionstherebetween, command module 64, which includes tip gap control system36, provides commands to controllers and actuators 44. These commandsenable independent operation of each proprotor system 20 a-20 dincluding tip gap adjustment, rotor speed and angular position. Flightcontrol computer 22 receives feedback and sensor measurements fromsensors 38, controllers, actuators 44 and proprotor systems 20 a-20 d.This feedback is processed by monitoring module 66, which includes tipgap monitoring system 34 and can supply correction data and otherinformation to command module 64 and/or controllers and actuators 44.Sensors 38, such as strain sensors, distance sensors, accelerometers,vibration sensors, location sensors, attitude sensors, speed sensors,environmental sensors, fuel sensors, temperature sensors and the likealso provide information to flight control computer 22 to furtherenhance autonomous control capabilities.

Some or all of the autonomous control capability of flight controlcomputer 22 can be augmented or supplanted by remote flight controlfrom, for example, remote system 54. Remote system 54 may include one ormore computing systems that may be implemented on general-purposecomputers, special purpose computers or other machines with memory andprocessing capability. Remote system 54 may be a microprocessor-basedsystem operable to execute program code in the form ofmachine-executable instructions. In addition, remote system 54 may beconnected to other computer systems via a proprietary encrypted network,a public encrypted network, the Internet or other suitable communicationnetwork that may include both wired and wireless connections. Remotesystem 54 communicates with flight control computer 22 via communicationlink 62 that may include both wired and wireless connections.

While operating remote control application 60, remote system 54 isconfigured to display information relating to one or more aircraft ofthe present disclosure on one or more flight data display devices 68.Remote system 54 may also include audio output and input devices such asa microphone, speakers and/or an audio port allowing an operator tocommunicate with other operators, a base station and/or a pilot onboardducted aircraft 10. Display device 68 may also serve as a remote inputdevice 70 if a touch screen display implementation is used, althoughother remote input devices such as a keyboard or joystick mayalternatively be used to allow an operator to provide control commandsto an aircraft being operated responsive to remote control.

Some or all of the autonomous and/or remote flight control of ductedaircraft 10 can be augmented or supplanted by onboard pilot flightcontrol from a pilot interface system 56 that includes one or morecomputing systems that communicate with flight control computer 22 viaone or more wired communication channels 72. Pilot system 56 preferablyincludes one or more cockpit display devices 74 configured to displayinformation to the pilot. Cockpit display device 74 may be configured inany suitable form including, for example, a display panel, a dashboarddisplay, an augmented reality display or the like. Pilot system 56 mayalso include audio output and input devices such as a microphone,speakers and/or an audio port allowing an onboard pilot to communicatewith, for example, air traffic control. Pilot system 56 also includes aplurality of user interface devices 76 to allow an onboard pilot toprovide control commands to ducted aircraft 10 including, for example, acontrol panel with switches or other inputs, mechanical control devicessuch as steering devices or sticks, voice control as well as othercontrol devices.

Referring to FIG. 4 in the drawings, a tip gap monitoring and controlsystem for a ducted aircraft such as ducted aircraft 10 in FIGS. 1A-1Fis schematically illustrated and generally designated 100. Tip gapmonitoring and control system of ducted aircraft 100 manages tip gap 102for proprotor system 104. A single proprotor system 104 of ductedaircraft 100 is shown for purposes of describing the functionality ofthe tip gap monitoring and control system, although it will beappreciated by one of ordinary skill in the art that the tip gapmonitoring and control system may manage the tip gaps of any number ofproprotor systems for ducted aircraft 100. Proprotor system 104 includesa proprotor hub 106 to which proprotor blades 108 are coupled. Proprotorblades 108 are surrounded by duct 110, which is supported by stators112. Proprotor system 104 is illustrated as including a horizontalstator and a vertical stator, although proprotor system 104 may includeany number of stators in any orientation. Ducted aircraft 100 alsoincludes flight control computer 114, which implements various systemsand modules of the tip gap monitoring and control system.

Tip gap monitoring system 116 provides in-flight monitoring of tip gap102 including the current or projected tip gap 102. Tip gap monitoringsystem 116 monitors tip gap 102 using sensors 118 coupled to proprotorsystem 104. Sensors 118 includes sensors 118 a coupled to proprotorblades 108. Sensors 118 a may be disposed inside or on the outer surfaceof proprotor blades 108. Sensors 118 b are coupled to duct 110 andsensors 118 c are coupled to stators 112. A portion of sensors 118 mayalso be coupled to proprotor hub 106. The number and placement ofsensors 118 depends on a variety of factors including the number ofproprotor blades 108, the number of stators 112, the design of proprotorsystem 104, sensor type, anticipated stresses, loads or flightconditions as well as other factors. Sensors 118 detect one or moreparameters of proprotor system 104 to generate sensor measurements 120.In some embodiments, sensors 118 may be strain gauges whose sensormeasurements 120 are proportional to the deflection, or strain,experienced by the components of proprotor system 104 to which they areattached. Strain gauges on proprotor blades 108, for example, may detectelongation of proprotor blades 108 due to centrifugal forces actingthereon. The strain gauges may also be used to detect loads on proprotorblades 108, duct 110 and/or stators 112 such as axial tension orcompression loads on stators 112. Non-limiting examples of strain gaugesthat may be coupled to proprotor system 104 include foil strain gauges,optical strain gauges or laser strain gauges. Accelerometers may also beused to measure the deflection of the components of proprotor system104. In some embodiments, sensors 118 may include blade pitch sensorsand sensor measurements 120 may indicate the pitch of proprotor blades108. Sensors 118 may also include optical gauges, laser sensors oraccelerometers to measure other parameters of proprotor system 104.

Referring to FIGS. 5A-5B in conjunction with FIG. 4 in the drawings,another implementation of tip gap monitoring system 116 is depicted inwhich sensors 118 are distance sensors 118 e, 118 f coupled to proprotorsystem 104 a. Sensor measurements 120 taken by distance sensors 118 e,118 f are tip gap distance measurements 102 a for each proprotor blade108 a. Non-limiting examples of distance sensors 118 e, 118 f includeHall Effect sensors, laser sensors or optical sensors. In someembodiments, distance sensors 118 f are magnets and distance sensors 118e are Hall Effect sensors that detect the proximity of magnets 118 f asproprotor blades 108 a rotate to generate tip gap distance measurements102 a. Any combination of the aforementioned strain gauges, distancesensors or other sensor types may be coupled to proprotor system 104 andutilized by tip gap monitoring system 116 to determine tip gap 102.

Referring back to FIG. 4, sensor measurements 120 are transmitted fromsensors 118 on proprotor system 104 to flight control computer 114. Tipgap monitoring system 116 includes a tip gap measurement module 122 todetermine tip gap 102 between duct 110 and proprotor blades 108 based onsensor measurements 120. Because tip gap 102 may differ for eachproprotor blade 108, tip gap measurement module 122 may determine arespective tip gap 102 for each proprotor blade 108 based on sensormeasurements 120. Assuming that tip gaps 102 differ for each proprotorblade 108, tip gap measurement module 122 may also determine a minimum,maximum, median or average tip gap 102 between duct 110 and proprotorblades 108. In some embodiments, sensors 118 may have nominal values andtip gap measurement module 122 may determine tip gap 102 by comparingsensor measurements 120 with the nominal values of sensors 118. In onenon-limiting implementation, tip gap measurement module 122 maycalculate tip gap 102 only if sensor measurements 120 differ from thenominal values for sensors 118 by a predetermined tolerance or noisethreshold to reduce the processing requirements of tip gap measurementmodule 122. For example, in an embodiment in which sensors 118 arestrain gauges, the strain measurements of sensor measurements 120 may beused to calculate strain if they exceed a threshold strain level. Thisthreshold strain level may be determined using analysis of the ductstructure, stress modeling, mapping and/or other methods. Thesethreshold strain levels may be tested and validated for refinement. Inother embodiments, tip gap measurement module 122 may continuouslycalculate tip gap 102 in real time to constantly monitor tip gap 102during flight.

In some embodiments, tip gap measurement module 122 determines tip gap102 based on the deflection or asymmetric loading experienced byproprotor system 104. In some implementations, tip gap measurementmodule 122 determines a structural deformity, or shape, of proprotorblades 108, duct 110 and/or stators 112 based on sensor measurements 120from sensors 118 such as strain gauges coupled to proprotor system 104.In particular, strain measurements can be used to calculate loads onproprotor blades 108, duct 110 and/or stators 112 to determine the shapeof these components based on the calculated loads. The structuraldeformity of proprotor blades 108, duct 110 and/or stators 112 may bedetermined through testing and calibration to flight loads, by computersimulation or by other numerical methods. In one example, the structuraldeformity or shape of duct 110 is determined using strain gauges onstators 112 to measure axial loads on stators 112 such as tension orcompression loads. In another example, strain measurements from straingauges on proprotor blades 108 may be used to determine any structuraldeformities of proprotor blades 108 due to centrifugal forces or otherloads. Once any structural deformities of proprotor system 104 aredetermined, tip gap measurement module 122 may then determine tip gap102 based on any calculated structural deformities of proprotor blades108, duct 110 and/or stators 112. For example, the derived shape ofproprotor blades 108, duct 110 and/or stators 112 may be compared to thenominal condition or shape of proprotor blades 108, duct 110 and/orstators 112, respectively, to determine tip gap 102 at the tips of eachproprotor blade 108. By determining tip gap 102 based on structuraldeformities of proprotor system 104, tip gap measurement module 122 maycalculate changes in tip gap 102 due to asymmetric loading of proprotorsystem 104, in-flight or imminent collisions with proprotor system 104such as bird strikes, load changes on proprotor system 104 due to flightconditions such as flight mode or flight maneuvers as well as otherevents and factors.

In some embodiments, a maneuver selection module 124 may utilize tip gap102, as determined by tip gap measurement module 122, to determine,select or preclude certain flight maneuvers that ducted aircraft 100 mayperform. For example, certain maneuvers may cause excessive structuraldeformity of the components of proprotor system 104, and maneuverselection module 124 may avoid such maneuvers if tip gap 102 is toosmall, thereby reducing the likelihood of collision between proprotorblades 108 and duct 110. Maneuver selection module 124 may also be usedto avoid maneuvers causing large increases in tip gap 102 that degradethe performance of ducted aircraft 100. Tip gap monitoring system 116may thus allow for safe envelope expansion for flight testing of ductedaircraft 100. Sensor measurements 120 may also indicate the absolute orrelative position of sensors 118, and therefore the position ofunderlying component(s) to which they are attached, and these positionalsensor measurements 120 may be used to determine tip gap 102. Forexample, sensors 118 may be accelerometers that are used to measuremovement of the portion(s) of proprotor system 104 to which theaccelerometers are coupled. The movement detected by the accelerometermay be used by tip gap measurement module 122 to determine anystructural deformities of proprotor system 104. The blade pitch ofproprotor blades 108, as indicated by sensor measurements 120 and/orflight control computer 114, may also be used by tip gap measurementmodule 122 to determine tip gap 102.

Tip gap monitoring system 116 may also include a tip gap determinationengine 126 that compares tip gap 102 calculated by tip gap measurementmodule 122 with a tip gap target 128 to determine a tip gap adjustmentdistance 130. Tip gap target 128 is the desired distance for tip gap 102and may be determined based on a number of factors such as the flightcondition of ducted aircraft 100 including the flight maneuver or flightmode currently being implemented by ducted aircraft 100. Tip gapadjustment distance 130 is the distance that tip gap 102 is to beadjusted to equal tip gap target 128. In some examples, tip gapadjustment distance 130 may be determined by calculating the differencebetween tip gap 102 and tip gap target 128. Tip gap adjustment distance130 may then be outputted by tip gap determination engine 126 for use byother systems or modules of flight control computer 114 including forpurposes of active or semi-active control of tip gap 102. In someembodiments, tip gap adjustment distance 130 may be outputted inresponse to tip gap 102 differing from tip gap target 128 by a tip gaptolerance threshold 132 so that tip gap adjustment distance 130 needonly be processed by other systems of flight control computer 114 whentip gap adjustment distance 130 is large enough to merit active orsemi-active control of tip gap 102.

The tip gap monitoring and control system of ducted aircraft 100includes tip gap control system 134 to actively or semi-actively controltip gap 102 using either or both of active blade tips 136 or activeinner duct surfaces 138. Referring to FIGS. 6A-6B in conjunction withFIG. 4 in the drawings, each proprotor blade 108 of proprotor system 104includes a respective active blade tip 136. Active blade tips 136 areslidably, or telescopically, coupled to open distal ends of main bodies140 of proprotor blades 108. Actuators 142 are disposed at proprotor hub106 and coupled to active blade tips 136 by spanwise links 144. Eachactuator 142 may include a solenoid for electromagnetic control ofactive blade tips 136. Each spanwise link 144 may be a cable, rod orother member connecting actuators 142 to active blade tips 136.

Actuators 142 move active blade tips 136 into a plurality of positionsincluding the extended position shown in FIG. 6A and the retractedposition shown in FIG. 6B. Actuators 142 may also move active blade tips136 into an infinite number of intermediate positions between theextended position shown in FIG. 6A and the retracted position shown inFIG. 6B. Active blade tips 136 move radially outward or inward along aspanwise axis of each proprotor blade 108. In the extended position,active blade tips 136 are extended from the open distal ends of mainbodies 140 of proprotor blades 108 to form tip gap 102 having a distanced₁. In the retracted position, active blade tips 136 are retracted intomain bodies 140 of proprotor blades 108 to form tip gap 102 having adistance D₁. Distance D₁ is greater than distance d₁ such that tip gap102 is larger when active blade tips 136 are in the retracted positionshown in FIG. 6B. Because each active blade tip 136 is associated with arespective actuator 142, active blade tips 136 may be independentlyactuated to permit nonuniform positioning of active blade tips 136. Forexample, only one or two of active blade tips 136 may be fully orpartially retracted while the remaining active blade tips of proprotorsystem 104 are extended. One scenario in which nonuniform positioning ofactive blade tips 136 may be useful is when proprotor blades 108experience nonuniform deflection or deformity loads. In someembodiments, each active blade tip 136 may be coupled to a respectivespring 146 that biases the active blade tips 136 into either theextended position or the retracted position. In one non-limitingexample, springs 146 may bias active blade tips 136 into the extendedposition and actuators 142 may be configured to pull active blade tips136 radially inward against the bias of springs 146 to provide rapidtwo-way movement of active blade tips 136. In other example, centrifugalforce during operation supplements or replaces the need for spring 146,in which case spring 146 may be useful to keep the system in tensionwhen proprotor system 104 is not in operation.

FIGS. 6C-6G illustrate alternative active blade tip configurations. InFIGS. 6C-6D, actuator 142 a for active blade tip 136 a is located at adistal portion of main body 140 a of proprotor blade 108 b, therebyshortening spanwise link 144 a between actuator 142 a and active bladetip 136 a. Actuator 142 a moves active blade tip 136 a between theextended position shown in FIG. 6C and the retracted position shown inFIG. 6D. Distance D₂ is greater than distance d₂ such that tip gap 102is larger when active blade tip 136 a is in the retracted position shownin FIG. 6D. In other embodiments, actuator 142 a may be located anywherewithin main body 140 a of proprotor blade 108 b including the root ormiddle portions of main body 140 a. In yet other embodiments, actuator142 a may abut active blade tip 136 a without the need for spanwise link144 a or may be located inside of active blade tip 136 a. Actuator 142 amay also be used in conjunction with a spring to bias active blade tip136 a in either the extended or retracted position, although centrifugalforce may instead be used to bias blade tip 136 a into the extendedposition.

In FIGS. 6E-6G, active blade tip 136 b is hingeably coupled to thedistal end of main body 140 b of proprotor blade 108 c at hinge 148.Actuator 142 b, which may be located in hinge 148 or elsewhere, movesactive blade tip 136 b between the extended position shown in FIG. 6Fand the retracted position shown in FIG. 6G. In the extended position,active blade tip 136 b is substantially coplanar with main body 140 b.In the retracted position, active blade tip 136 b forms an angle θ₁ ofless than 180 degrees with main body 140 b. In the illustratedembodiment, angle θ₁ is an obtuse angle and active blade tip 136 b isnon-coplanar with main body 140 b in the retracted position. When activeblade tip 136 b is in the extended position, tip gap 102 has a distanced₃, which is shorter than distance D₃ of tip gap 102 when active bladetip 136 b is in the retracted position.

Referring back to FIG. 4, tip gap control system 134 includes a bladelength control module 150 to actively or semi-actively actuate activeblade tips 136 to maintain a desired tip gap 102 for safety andincreased performance. Blade length control module 150 generates bladetip actuator commands 152 to control the position of active blade tips136. Blade tip actuator commands 152 are transmitted from flight controlcomputer 114 to proprotor system 104 so that actuators 142 extend orretract active blade tips 136 to control tip gap 102 between duct 110and proprotor blades 108. In some embodiments, blade tip actuatorcommands 152 include tip gap adjustment distance 130 determined by tipgap monitoring system 116. Upon receiving blade tip actuator commands152, actuators 142 may then move active blade tips 136 by tip gapadjustment distance 130 so that tip gap 102 is substantially equal totip gap target 128. In certain embodiments, blade length control module150 generates blade tip actuator commands 152 in response to receivingtip gap adjustment distance 130 from tip gap monitoring system 116.While blade length control module 150 may continuously adjust activeblade tips 136 in real time to match tip gap target 128 by sending tipgap adjustment distance 130 to actuators 142 regardless of magnitude, inother embodiments blade length control module 150 may generate blade tipactuator commands 152 in response to tip gap adjustment distance 130exceeding a tip gap adjustment distance threshold to avoidmicro-adjustments below such a threshold. Blade tip actuator commands152 may extend or retract active blade tips 136 uniformly or may includeone or more blade-specific blade tip actuator commands, eachcorresponding to a respective one of active blade tips 136 fornonuniform positioning. Thus, blade length control module 150 may extendor retract all or a portion of active blade tips 136 at any given time.For example, blade tip actuator commands 152 may include ablade-specific blade tip actuator command that extends only one ofactive blade tips 136 if such proprotor blade, for example, is shorterthan the other proprotor blades due to deflection or other reasons.

While tip gap control system 134 may actively control tip gap 102 usingactive blade tips 136 during all portions of a flight, tip gap controlsystem 134 may also semi-actively control active blade tips 136.Semi-active control of active blade tips 136 may be particularly usefulin response to events such as bird strikes or severe maneuvers of ductedaircraft 100. In some semi-active implementations, blade tip actuatorcommands 152 may include a retract command or an extend command forproprotor blades 108, and actuators 142 may move active blade tips 136by a predetermined distance in response to receiving the retract orextend command. For example, blade length control module 150 may sendblade tip actuator commands 152 that retract active blade tips 136 by apredetermined distance in response to detecting an imminent or actualcollision such as a bird strike with proprotor system 104 during flight.Blade length control module 150 may also generate blade tip actuatorcommands 152 that retract active blade tips 136 by a predetermineddistance in response to tip gap monitoring system 116 detecting astructural deformity of proprotor system 104. In one non-limitingexample, active blade tips 136 may be commanded to retract only if thestructural deformity meets or exceeds a structural deformity threshold.

Blade length control module 150 may also generate blade tip actuatorcommands 152 based on the flight condition of ducted aircraft 100. Forexample, blade length control module 150 may generate blade tip actuatorcommands 152 that retract active blade tips 136 by a predetermined orcalculated distance if ducted aircraft 100 executes a flight maneuver154 that has been predetermined to subject proprotor system 104 toexcessive loading. Flight maneuver 154 may be detected by flight controlcomputer 114 using a maneuver detection module 156. Maneuver detectionmodule 156 may use sensors 118, pilot inputs, remote inputs or otherparameters to determine flight maneuver 154 being executed by ductedaircraft 100.

Blade length control module 150 may also generate blade tip actuatorcommands 152 based on the flight mode of ducted aircraft 100. Examplesof such flight modes include the VTOL flight mode, the forward flightmode and the conversion flight mode described in FIGS. 1A-1F. Therotational speed of proprotor system 104 is lower in the forward flightmode than in the VTOL flight mode, which expands tip gap 102 in theforward flight mode. Blade length control module 150 may eithersupplement or compensate for the naturally higher tip gap 102 in theforward flight mode. Thus, blade length control module 150 may generateblade tip actuator commands 152 that move active blade tips 136 betweenthe retracted and extended positions in response to ducted aircraft 100converting between the VTOL flight mode and the forward flight mode. Forexample, blade length control module 150 may generate blade tip actuatorcommands 152 that move active blade tips 136 by a calculated orpredetermined amount from the retracted position to the extendedposition in response to ducted aircraft 100 converting from the VTOLflight mode to the forward flight mode, thus compensating for thenaturally higher tip gap in the forward flight mode. In another example,blade length control module 150 may generate blade tip actuator commands152 that move active blade tips 136 by a calculated or predeterminedamount from the extended position to the retracted position in responseto ducted aircraft 100 converting from the VTOL flight mode to theforward flight mode, thus supplementing tip gap 102 to provideadditional safety. Because the blade pitch of proprotor blades 108changes when converting from the VTOL flight mode to the forward flightmode, blade length control module 150 may also generate blade tipactuator commands 152 based on the blade pitch of proprotor blades 108.Because proprotor system 104 typically experiences higher loading in theconversion flight mode, blade length control module 150 may alsogenerate blade tip actuator commands 152 that retract active blade tips136 by a calculated or predetermined amount when ducted aircraft 100 isin the conversion flight mode between the VTOL flight mode and theforward flight mode.

Additionally or alternatively, tip gap control system 134 may controltip gap 102 using active inner duct surfaces 138. Referring to FIGS.7A-7C in conjunction with FIG. 4 of the drawings, duct 110 has an innersurface 158 that forms a circumferential cavity, or slot, 160. Activeinner duct surfaces 138 are actuated segmented surfacescircumferentially disposed on inner surface 158 of duct 110. The tips ofproprotor blades 108 pass adjacent to a blade pass band 162 on innersurface 158 of duct 110. Active inner duct surfaces 138 are disposedalong blade pass band 162 and slidably coupled to duct 110 at cavity160.

Each active inner duct surface 138 is coupled to a respective actuator164. Actuators 164 move active inner duct surfaces 138 between theextended position shown in FIG. 7B and the retracted position shown inFIG. 7C as well as intermediate positions therebetween. Actuators 164are disposed inside of duct 110 and may include a solenoid, springs orother actuating components. Active inner duct surfaces 138 aresubstantially flush with inner surface 158 of duct 110 in the extendedposition. Active inner duct surfaces 138 slide into cavity 160 in theretracted position to increase tip gap 102 at blade pass band 162.Distance D₄ is greater than distance d₄ such that tip gap 102 is largerwhen active inner duct surfaces 138 are in the retracted position shownin FIG. 7C. Using active inner duct surfaces 138, duct 110 may beconstricted or expanded at blade pass band 162 while maintaining theoverall smoothness of inner surface 158. In some embodiments, activeinner duct surfaces 138 may be extended beyond the position shown inFIG. 7B so that active inner duct surfaces 138 act as bumpouts tofurther reduce tip gap 102. Because each active inner duct surface 138is coupled to a respective actuator 164, active inner duct surfaces 138may be independently actuated to permit nonuniform positioning of activeinner duct surfaces 138.

FIGS. 7D-7G illustrate alternative configurations of the active innerduct surfaces. In FIGS. 7D-7E, active inner duct surface 138 a ishingeably coupled to inner surface 158 a of duct 110 a at cavity 160 avia hinge 166. Active inner duct surface 138 a may be a flap or thinsheet plate. Actuator 164 a rotates active inner duct surface 138 abetween the extended position shown in FIG. 7D and the retractedposition shown in FIG. 7E. In the extended position, active inner ductsurface 138 a is substantially flush with inner surface 158 a of duct110 a. In the retracted position, active inner duct surface 138 a isrotated into cavity 160 a. Distance D₅ is greater than distance d₅ suchthat tip gap 102 is larger when active inner duct surface 138 a is inthe retracted position shown in FIG. 7E. In some embodiments, activeinner duct surface 138 a may be extended further than the extendedposition shown in FIG. 7D to further reduce tip gap 102.

In FIGS. 7F-7G, active inner duct surface 138 b is fillable with fluid.Fluid-filled active inner duct surface 138 b is disposed in cavity 160b. The fluid inside fluid-filled active inner duct surface 138 b may bea liquid or a gas. Fluid-filled active inner duct surface 138 b may betubular or have any cross-sectional shape. Actuator 164 b is a pump thatinjects and removes fluid to and from active inner duct surface 138 b.Actuator pump 164 b fills active inner duct surface 138 b with fluid inthe extended position such that fluid-filled active inner duct surface138 b is substantially flush with inner surface 158 b of duct 110 b asshown in FIG. 7F. Actuator pump 164 b removes fluid from fluid-filledactive inner duct surface 138 b in the retracted position as shown inFIG. 7G. Thus, active inner duct surface 138 b is inflated in theextended position and deflated in the retracted position. Distance D₆ isgreater than distance d₆ such that tip gap 102 is larger whenfluid-filled active inner duct surface 138 b is in the retractedposition shown in FIG. 7G.

Referring back to FIG. 4, tip gap control system 134 includes an innerduct surface control module 168 to actively or semi-actively actuateactive inner duct surfaces 138 to maintain a desired tip gap 102 forsafety and increased performance. Inner duct surface control module 168generates inner duct surface actuator commands 170 to control theposition of active inner duct surfaces 138. Inner duct surface actuatorcommands 170 are transmitted from flight control computer 114 toproprotor system 104 so that actuators 164 extend or retract activeinner duct surfaces 138 to control tip gap 102 between duct 110 andproprotor blades 108. In some embodiments, inner duct surface actuatorcommands 170 include tip gap adjustment distance 130 determined by tipgap monitoring system 116. Upon receiving inner duct surface actuatorcommands 170, actuators 164 may move active inner duct surfaces 138 bytip gap adjustment distance 130 so that tip gap 102 is substantiallyequal to tip gap target 128. In certain embodiments, inner duct surfacecontrol module 168 generates inner duct surface actuator commands 170 inresponse to receiving tip gap adjustment distance 130 from tip gapmonitoring system 116. While inner duct surface control module 168 maycontinuously adjust active inner duct surfaces 138 in real time to matchtip gap target 128 by sending tip gap adjustment distance 130 toactuators 164 regardless of magnitude, in other embodiments inner ductsurface control module 168 may generate inner duct surface actuatorcommands 170 in response to tip gap adjustment distance 130 exceeding atip gap adjustment distance threshold to avoid micro-adjustments belowsuch a threshold. Inner duct surface actuator commands 170 may extend orretract active inner duct surfaces 138 uniformly or may include one ormore active inner duct surface-specific actuator commands eachcorresponding to a respective one of active inner duct surfaces 138 fornonuniform positioning. Thus, inner duct surface control module 168 mayextend or retract all or a portion of active inner duct surfaces 138 atany given time. For example, inner duct surface actuator commands 170may include an active inner duct surface-specific actuator command thatretracts only one of active inner duct surfaces 138 if the portion ofduct 110 at which the active inner duct surface is disposed has beendeformed.

While tip gap control system 134 may actively control tip gap 102 usingactive inner duct surfaces 138 during all portions of a flight, tip gapcontrol system 134 may also semi-actively control active inner ductsurfaces 138. Semi-active control of active inner duct surfaces 138 maybe particularly useful in response to events such as bird strikes orsevere maneuvers of ducted aircraft 100. In some semi-activeimplementations, inner duct surface actuator commands 170 may include aretract command or an extend command for duct 110, and actuators 164 maymove active inner duct surfaces 138 by a predetermined distance inresponse to receiving the retract or extend command. For example, innerduct surface control module 168 may send inner duct surface actuatorcommands 170 that retract active inner duct surfaces 138 by apredetermined distance in response to detecting an actual or imminentcollision such as a bird strike with proprotor system 104 during flight.Inner duct surface control module 168 may also generate inner ductsurface actuator commands 170 that retract active inner duct surfaces138 by a predetermined distance in response to tip gap monitoring system116 detecting a structural deformity of proprotor system 104. In onenon-limiting example, active inner duct surfaces 138 may be commanded toretract only if the structural deformity meets or exceeds a structuraldeformity threshold.

Inner duct surface control module 168 may also generate inner ductsurface actuator commands 170 based on the flight condition of ductedaircraft 100. For example, inner duct surface control module 168 maygenerate inner duct surface actuator commands 170 that retract activeinner duct surfaces 138 by a predetermined or calculated distance ifducted aircraft 100 executes a flight maneuver 154, as detected bymaneuver detection module 156, which has been predetermined to subjectproprotor system 104 to excessive loading. Inner duct surface controlmodule 168 may also generate inner duct surface actuator commands 170based on the flight mode of ducted aircraft 100. The rotational speed ofproprotor system 104 is lower in the forward flight mode than in theVTOL flight mode, which expands tip gap 102 in the forward flight mode.Inner duct surface control module 168 may either supplement orcompensate for the naturally higher tip gap 102 in the forward flightmode. Thus, inner duct surface control module 168 may generate innerduct surface actuator commands 170 that move active inner duct surfaces138 between the retracted and extended positions in response to ductedaircraft 100 converting between the VTOL flight mode and the forwardflight mode. For example, inner duct surface control module 168 maygenerate inner duct surface actuator commands 170 that move active innerduct surfaces 138 by a calculated or predetermined amount from theretracted position to the extended position in response to ductedaircraft 100 converting from the VTOL flight mode to the forward flightmode, thus compensating for the naturally higher tip gap in the forwardflight mode. In another example, inner duct surface control module 168may generate inner duct surface actuator commands 170 that move activeinner duct surfaces 138 by a calculated or predetermined amount from theextended position to the retracted position in response to ductedaircraft 100 converting from the VTOL flight mode to the forward flightmode, thus supplementing tip gap 102 to provide additional safety.

Because the blade pitch of proprotor blades 108 changes when convertingfrom the VTOL flight mode to the forward flight mode, inner duct surfacecontrol module 168 may also generate inner duct surface actuatorcommands 170 based on the blade pitch of proprotor blades 108. Becauseproprotor system 104 typically experiences higher loading in theconversion flight mode, inner duct surface control module 168 may alsogenerate inner duct surface actuator commands 170 that retract activeinner duct surfaces 138 by a calculated or predetermined amount whenducted aircraft 100 is in the conversion flight mode between the VTOLflight mode and the forward flight mode. The systems and modules shownas part of flight control computer 114 may be interchangeable with oneanother. In certain embodiments, flight control computer 114 may includeor implement only a portion of the systems and modules shown in FIG. 4in any combination.

Referring additionally to FIGS. 8A-8H in the drawings, a sequentialflight-operating scenario of ducted aircraft 100 including proprotorsystems 104 and flight control computer 114 is depicted. Proprotorsystems 104 include forward-port, forward-starboard, aft-port andaft-starboard proprotor systems similar to ducted aircraft 10 in FIGS.1A-1F. As best seen in FIG. 8A, ducted aircraft 100 is positioned on theground prior to takeoff. When ducted aircraft 100 is ready for amission, flight control computer 114 commences operations to provideflight control to ducted aircraft 100 which may be onboard pilot flightcontrol, remote flight control, autonomous flight control or acombination thereof. For example, it may be desirable to utilize onboardpilot flight control during certain maneuvers such as takeoff andlanding but rely on autonomous flight control during hover, high speedforward flight and/or transitions between wing-borne flight andthrust-borne flight.

As best seen in FIG. 8B, ducted aircraft 100 has performed a verticaltakeoff and is engaged in thrust-borne lift. As illustrated, theproprotor assemblies of each proprotor system 104 are rotating in thesame horizontal plane forming a two-dimensional distributed thrust arrayof four proprotor systems. As the longitudinal axis and the lateral axisof ducted aircraft 100 are both in the horizontal plane, ducted aircraft100 has a level flight attitude. During hover, flight control computer114 utilizes individual variable speed and blade pitch controlcapability of proprotor systems 104 to control flight dynamics tomaintain hover stability and to provide pitch, roll and yaw authorityfor ducted aircraft 100. More specifically, as each proprotor system 104is independently controllable, operational changes to certain proprotorsystems 104 enable pitch, roll and yaw control of ducted aircraft 100during VTOL operations.

For example, by changing the thrust output of the forward proprotorsystems relative to the aft proprotor systems, pitch control isachieved. As another example, by changing the thrust output of the portproprotor systems relative to the starboard proprotor systems, rollcontrol is achieved. Changing the relative thrust outputs of the variousproprotor systems 104 may be accomplished using differential rotor speedcontrol, that is, increasing the rotor speed of some proprotor systemsrelative to the rotor speed of other proprotor systems and/or decreasingthe rotor speed of some proprotor systems relative to the rotor speed ofother proprotor systems. Changing the relative thrust outputs of thevarious proprotor systems 104 may be accomplished using collective bladepitch. Yaw control or torque balancing of ducted aircraft 100 duringVTOL operations may be accomplished by changing the torque output ofcertain proprotor systems 104. For example, the forward-port andaft-starboard proprotor systems may have clockwise rotating proprotorassemblies while the forward-starboard and aft-port proprotor systemsmay have counterclockwise rotating proprotor assemblies. In thisexample, by changing the torque output of the forward-port andaft-starboard proprotor systems relative to the forward-starboard andaft-port proprotor systems, yaw control is achieved. Changing therelative torque outputs of the various proprotor systems 104 may beaccomplished using differential rotor speed control.

During hover, ducted aircraft 100 may experience crosswinds that causeturbulent flow through proprotor systems 104. This turbulent flowsubjects proprotor systems 104 to deforming loads, which affect tip gap102. Tip gap monitoring system 116 detects changes to tip gap 102 causedby crosswinds. If tip gap 102 becomes unacceptably small due to theresulting deforming loads, tip gap control system 134 repositions activeblade tips 136 and/or active inner duct surfaces 138 to enlarge tip gap102.

Returning to the sequential flight-operating scenario of ducted aircraft100, after vertical ascent to the desired elevation, ducted aircraft 100may begin the transition from thrust-borne lift to wing-borne lift. Asbest seen from the progression of FIGS. 8B-8D, the angular positions ofproprotor systems 104 are changed by a pitch down rotation to transitionducted aircraft 100 from the VTOL flight mode toward the forward flightmode. As seen in FIG. 8C, proprotor systems 104 have been collectivelyinclined about 45 degrees pitch down. In the conversion orientations ofducted aircraft 100, a portion of the thrust generated by proprotorsystems 104 provides lift while a portion of the thrust generated byproprotor systems 104 urges ducted aircraft 100 to accelerate in theforward direction such that the forward airspeed of ducted aircraft 100increases allowing the wings of ducted aircraft 100 to offload a portionand eventually all of the lift requirement from proprotor systems 104.Proprotor systems 104 may be particularly susceptible to deforming loadsin the conversion flight mode shown in FIG. 8C. Tip gap monitoringsystem 116 detects any structural deformities to proprotor systems 104during the transition from the VTOL flight mode to the forward flightmode. Tip gap control system 134 repositions active blade tips 136and/or active inner duct surfaces 138 in response to any structuraldeformities that unacceptably reduce tip gap 102 during the conversionflight mode to ensure that no collisions occur between proprotor blades108 and duct 110.

As best seen in FIGS. 8D-8E, proprotor systems 104 have beencollectively inclined about 90 degrees pitch down such that theproprotor assemblies are rotating in vertical planes providing forwardthrust for ducted aircraft 100 while the wings provide lift. Even thoughthe conversion from the VTOL flight mode to the forward flight mode ofducted aircraft 100 has been described as progressing with collectivepitch down rotation of proprotor systems 104, in other implementations,all proprotor systems 104 need not be operated at the same time or atthe same rate. As forward flight with wing-borne lift requiressignificantly less thrust than VTOL flight with thrust-borne lift, theoperating speed of some or all of proprotor systems 104 may be reducedparticularly in embodiments having collective pitch control. This RPMreduction in the forward flight mode tends to increase tip gap 102. Tipgap control system 134 may decrease tip gap 102 in the forward flightmode by extending active blade tips 136 and/or active inner ductsurfaces 138 to compensate for the enlarged tip gap 102. In otherembodiments, tip gap control system 134 may further increase tip gap 102in the forward flight mode by retracting active blade tips 136 and/oractive inner duct surfaces 138 since tip gap 102 may be less critical toflight efficiency in the forward flight mode than in the VTOL flightmode. Tip gap control system 134 may increase or decrease tip gap 102 inresponse to flight control computer 114 signaling a change in flightmode. Because the blade pitch of proprotor blades 108 changes whenconverting from the VTOL flight mode to the forward flight mode, tip gapcontrol system 134 may also increase or decrease tip gap 102 in responseto flight control computer 114 or sensors 118 signaling a change inblade pitch. Proprotor systems 104 are susceptible to collisions such asa collision 174 with a bird 176. Such collisions can cause proprotorsystems 104 to structurally deform. Tip gap monitoring system 116detects any structural deformities to proprotor systems 104 caused byactual or imminent collisions during flight. Tip gap control system 134repositions active blade tips 136 and/or active inner duct surfaces 138in response to any structural deformities caused by collision 174 thatunacceptably reduce tip gap 102.

In certain embodiments, some of proprotor systems 104 of ducted aircraft100 could be shut down during forward flight. In the forward flightmode, the independent rotor speed control provided by flight controlcomputer 114 over each proprotor system 104 may provide yaw authorityfor ducted aircraft 100. For example, by changing the thrust output ofeither or both port proprotor systems relative to starboard proprotorsystems, yaw control is achieved. Changing the relative thrust outputsof the various proprotor systems 104 may be accomplished usingdifferential rotor speed control. Changing the relative thrust outputsof the various proprotor systems 104 may also be accomplished usingcollective pitch control. In the forward flight mode, pitch and rollauthority is preferably provided by the ailerons and/or elevators on thewings and/or tail assembly of ducted aircraft 100.

As ducted aircraft 100 approaches its destination, ducted aircraft 100may begin its transition from wing-borne lift to thrust-borne lift. Asbest seen from the progression of FIGS. 8E-8G, the angular positions ofproprotor systems 104 are changed by a pitch up rotation to transitionducted aircraft 100 from the forward flight mode toward the VTOL flightmode. As seen in FIG. 8F, proprotor systems 104 have been collectivelyinclined about 45 degrees pitch up. In the conversion orientations ofducted aircraft 100, a portion of the thrust generated by proprotorsystems 104 begins to provide lift for ducted aircraft 100 as theforward airspeed decreases and the lift producing capability of thewings of ducted aircraft 100 decreases. As best seen in FIG. 8G,proprotor systems 104 have been collectively inclined about 90 degreespitch up such that the proprotor assemblies are rotating in thehorizontal plane providing thrust-borne lift for ducted aircraft 100.Even though the conversion from the forward flight mode to the VTOLflight mode of ducted aircraft 100 has been described as progressingwith collective pitch up rotation of proprotor systems 104, in otherimplementations, all proprotor systems 104 need not be operated at thesame time or at the same rate. Once ducted aircraft 100 has completedthe transition to the VTOL flight mode, ducted aircraft 100 may commenceits vertical descent to a surface. As best seen in FIG. 8H, ductedaircraft 100 has landed at the destination location.

Referring to FIGS. 9A-9C in the drawings, methods for monitoring andcontrolling a tip gap for a ducted aircraft are illustrated asflowcharts 200, 202, 204. In FIG. 9A, a method for monitoring a tip gapfor a ducted aircraft includes detecting one or more parameters of aproprotor system including a duct and a plurality of proprotor blades toform a plurality of sensor measurements (step 206). The method includestransmitting the sensor measurements from a plurality of sensors coupledto the proprotor system to a flight control computer (step 208). Themethod includes determining a tip gap distance based on the sensormeasurements (step 210). The method also includes determining a tip gapadjustment distance based on the tip gap distance and a tip gap target(step 212).

In FIG. 9B, a method for controlling a tip gap for a ducted aircraftincludes generating a blade tip actuator command (step 214). The methodincludes transmitting the blade tip actuator command to a proprotorsystem including a duct and a plurality of proprotor blades, theproprotor blades including active blade tips (step 216). The method alsoincludes moving at least one of the active blade tips between aretracted position and an extended position in response to the blade tipactuator command, thereby controlling the tip gap between the proprotorblades and the duct (step 218). In FIG. 9C, a method for controlling atip gap for a ducted aircraft includes generating an inner duct surfaceactuator command (step 220). The method includes transmitting the innerduct surface actuator command to a proprotor system including a duct anda plurality of proprotor blades, the duct including a plurality ofactive inner duct surfaces (step 222). The method also includes movingat least one of the active inner duct surfaces between a retractedposition and an extended position in response to the inner duct surfaceactuator command, thereby controlling the tip gap between the proprotorblades and the duct (step 224).

The flowcharts and block diagrams in the different depicted embodimentsillustrate the architecture, functionality, and operation of somepossible implementations of apparatus, methods and computer programproducts. In this regard, each block in the flowchart or block diagramsmay represent a module, segment, or portion of code, which comprises oneor more executable instructions for implementing the specified functionor functions. In some alternative implementations, the function orfunctions noted in the block may occur out of the order noted in thefigures. For example, in some cases, two blocks shown in succession maybe executed substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved.

Referring to FIGS. 10A-10E in the drawings, a ducted aircraftimplementing a passive tip gap control system is schematicallyillustrated and generally designated 300. Proprotor system 302 is one ofa plurality of proprotor systems of ducted aircraft 300. Similar toducted aircraft 10 in FIGS. 1A-1F, the proprotor systems of ductedaircraft 300 including proprotor system 302 are rotatable between thevertical orientation of the forward flight mode shown in FIGS. 10A-10Cand the horizontal orientation of the VTOL flight mode shown in FIGS.10D-10E. Proprotor system 302 includes proprotor blades 304 surroundedby duct 306. A pitch control assembly 308 is coupled to proprotor blades304 to change the collective pitch of proprotor blades 304 about pitchchange axis 310. Ducted aircraft 300 relies on thrust-borne lift in theVTOL flight mode and wing-borne lift in the forward flight mode.Proprotor blades 304 have a lower collective pitch in the VTOL flightmode as shown in FIGS. 10D-10E than in the forward flight mode as shownin FIGS. 10B-10C. In addition, proprotor blades 304 operate at twodifferent rotational speeds in the two primary flight modes of ductedaircraft 300. More particularly, proprotor system 302 has a higherrotational speed in the VTOL flight mode than the forward flight mode.Due to a decrease in centrifugal force as well as other factors, thelower rotational speed of proprotor system 302 in the forward flightmode increases tip gap 312 in the forward flight mode relative to tipgap 312 in the VTOL flight mode. The passive tip gap control system ofducted aircraft 300 passively ties the length of proprotor blades 304 tothe blade pitch of proprotor blades 304 to accentuate or compensate forincreased tip gap 312 in the forward flight mode.

Proprotor blades 304 are extendable along pitch change axis 310 into aplurality of positions including the extended position shown in FIGS.10B-10C and the retracted position shown in FIGS. 10D-10E as well as aninfinite number of intermediate positions therebetween. Moreparticularly, each proprotor blade 304 includes a blade tip extension314 extendable into the extended position shown in FIGS. 10B-10C andretractable into the retracted position shown in FIGS. 10D-10E. Bladetip extensions 314 are slidably, or telescopically, coupled to the opendistal ends of main bodies 316 of proprotor blades 304. Blade tipextensions 314 are retracted into main bodies 316 of proprotor blades304 in the retracted position and extended from the open distal ends ofmain bodies 316 of proprotor blades 304 in the extended position,thereby increasing tip gap 312 in the retracted position and decreasingtip gap 312 in the extended position.

Each blade tip extension 314 is associated with a respective pitch-spanconverter 318. Each pitch-span converter 318 is coupled to andinterposed between pitch control assembly 308 and a respective blade tipextension 314. Pitch-span converters 318 are located at proprotor hub320 of proprotor system 302, although pitch-span converters 318 may belocated anywhere on proprotor system 302 including within main bodies316 of proprotor blades 304. Each pitch-span converter 318 is coupled toa respective blade tip extension 314 by a spanwise link 322 such as acable or rod disposed inside a respective proprotor blade 304.Pitch-span converters 318 convert the collective pitch of proprotorblades 304 about pitch change axis 310 into the position of blade tipextensions 314 along pitch change axis 310. In some embodiments, eachpitch-span converter 318 may include a screw mechanism such as a helicalthreaded screw or a pulley to convert the axial motion of pitch controlassembly 308 along mast 324 into the spanwise and radially extendablemotion of blade tip extensions 314. In embodiments in which eachspanwise link 322 is a cable, the cable may pull a respective blade tipextension 314 into the retracted position against the bias of arespective spring 326 and/or centrifugal force during operation.Centrifugal force during operation may supplement or replace the needfor spring 326, in which case spring 326 may be useful to keep thesystem in tension when proprotor system 302 is not in operation. Inalternative embodiments, blade tip extensions 314 may instead behingeably coupled to the distal ends of main bodies 316 of proprotorblades 304 in a similar manner to that described for active blade tips136 b in FIGS. 6E-6G. In such alternative embodiments, blade tipextensions 314 are substantially coplanar with main bodies 316 ofproprotor blades 304 in the extended position and form an angle of lessthan 180 degrees with main bodies 316 of proprotor blades 304 in theretracted position, thereby increasing tip gap 312 in the retractedposition and decreasing tip gap 312 in the extended position.

Blade tip extensions 314 change between the extended position of FIGS.10B-10C and the retracted position of FIGS. 10D-10E based on thecollective pitch of proprotor blades 304 to control tip gap 312. Asdescribed above, proprotor blades 304 have a lower collective pitch inthe VTOL flight mode and a higher collective pitch in the forward flightmode. Thus, blade tip extensions 314 change between the retractedposition and the extended position based on the flight mode of ductedaircraft 300. FIGS. 10D-10E show proprotor system 302 in the VTOL flightmode, in which proprotor blades 304 have a lower collective pitch andblade tip extensions 314 are in the retracted position. FIGS. 10B-10Cshow proprotor system 302 in the forward flight mode, in which proprotorblades 304 have a higher collective pitch and blade tip extensions 314are in the extended position to compensate for the shortened span ofmain bodies 316 of proprotor blades 304 in the forward flight mode.Thus, tip gap 312 remains substantially the same in both flight modes.Alternatively, blade tip extensions 314 may extend at the lowercollective pitch in the VTOL flight mode and retract at the highercollective pitch of the forward flight mode to increase tip gap 312 inthe forward flight mode. Increasing tip gap 312 in this manner may beuseful because tip gap 312 is not as critical to performance in theforward flight mode. Increasing tip gap 312 in the forward flight modemay also be used as a safety measure to guard against large deformationsof duct 306 due to bird strikes or other collisions, which are morelikely to occur and incur more damage in the forward flight mode.

Referring additionally to FIGS. 11A-11H in the drawings, a sequentialflight-operating scenario of ducted aircraft 300 including proprotorsystems 302 is depicted. Proprotor systems 302 include forward-port,forward-starboard, aft-port and aft-starboard proprotor systems similarto ducted aircraft 10 in FIGS. 1A-1F. As best seen in FIG. 11A, ductedaircraft 300 is positioned on the ground prior to takeoff. When ductedaircraft 300 is ready for a mission, the flight control computercommences operations to provide flight control to ducted aircraft 300which may be onboard pilot flight control, remote flight control,autonomous flight control or a combination thereof. For example, it maybe desirable to utilize onboard pilot flight control during certainmaneuvers such as takeoff and landing but rely on autonomous flightcontrol during hover, high speed forward flight and/or transitionsbetween wing-borne flight and thrust-borne flight.

As best seen in FIG. 11B, ducted aircraft 300 has performed a verticaltakeoff and is engaged in thrust-borne lift. As illustrated, theproprotor assemblies of each proprotor system 302 are rotating in thesame horizontal plane forming a two-dimensional distributed thrust arrayof four proprotor systems. As the longitudinal axis and the lateral axisof ducted aircraft 300 are both in the horizontal plane, ducted aircraft300 has a level flight attitude. During hover, the flight controlcomputer utilizes individual variable speed and blade pitch controlcapability of proprotor systems 302 to control flight dynamics tomaintain hover stability and to provide pitch, roll and yaw authorityfor ducted aircraft 300. More specifically, as each proprotor system 302is independently controllable, operational changes to certain proprotorsystems 302 enable pitch, roll and yaw control of ducted aircraft 300during VTOL operations.

Returning to the sequential flight-operating scenario of ducted aircraft300, after vertical assent to the desired elevation, ducted aircraft 300may begin the transition from thrust-borne lift to wing-borne lift. Asbest seen from the progression of FIGS. 11B-11D, the angular positionsof proprotor systems 302 are changed by a pitch down rotation totransition ducted aircraft 300 from the VTOL flight mode toward theforward flight mode. As seen in FIG. 11C, proprotor systems 302 havebeen collectively inclined about 45 degrees pitch down. In theconversion flight mode of ducted aircraft 300, a portion of the thrustgenerated by proprotor systems 302 provides lift while a portion of thethrust generated by proprotor systems 302 urges ducted aircraft 300 toaccelerate in the forward direction such that the forward airspeed ofducted aircraft 300 increases allowing the wings of ducted aircraft 300to offload a portion and eventually all of the lift requirement fromproprotor systems 302. As best seen in FIGS. 11D-11E, proprotor systems302 have been collectively inclined about 90 degrees pitch down suchthat the proprotor assemblies are rotating in vertical planes providingforward thrust for ducted aircraft 300 while the wings provide lift.

Proprotor systems 302 are designed to have a minimum tip gap 312 in theVTOL flight mode to maximize duct performance. As ducted aircraft 300transitions from the VTOL flight mode in FIG. 11B to the forward flightmode in FIG. 11D, the collective pitch of proprotor blades 304 increasesand the RPMs of proprotor systems 302 decrease, thereby enlarging tipgap 312 in the forward flight mode. The passive tip gap control systemextends blade tip extensions 314 as the collective pitch of proprotorblades 304 increases into the forward flight mode to compensate for thisenlarged tip gap 312. In other embodiments, the passive tip gap controlsystem may retract blade tip extensions 314 as the collective pitch ofproprotor blades 304 increases into the forward flight mode to provideadditional safety since tip gap 312 is less critical for performance inthe forward flight mode.

As ducted aircraft 300 approaches its destination, ducted aircraft 300may begin its transition from wing-borne lift to thrust-borne lift. Asbest seen from the progression of FIGS. 11E-11G, the angular positionsof proprotor systems 302 are changed by a pitch up rotation totransition ducted aircraft 300 from the forward flight mode to the VTOLflight mode. As seen in FIG. 11F, proprotor systems 302 have beencollectively inclined about 45 degrees pitch up. In the conversionorientations of ducted aircraft 300, a portion of the thrust generatedby proprotor systems 302 begins to provide lift for ducted aircraft 300as the forward airspeed decreases and the lift producing capability ofthe wings of ducted aircraft 300 decreases. As best seen in FIG. 11G,proprotor systems 302 have been collectively inclined about 90 degreespitch up such that the proprotor assemblies are rotating in thehorizontal plane providing thrust-borne lift for ducted aircraft 300. Asducted aircraft 300 transitions from the forward flight mode in FIG. 11Eto the VTOL flight mode in FIG. 11G, the collective pitch of proprotorblades 304 decreases and the RPMs of proprotor systems 302 increase,thereby constricting tip gap 312 in the VTOL flight mode. The passivetip gap control system retracts blade tip extensions 314 as thecollective pitch of proprotor blades 304 decreases into the VTOL flightmode to avoid any collisions between proprotor blades 304 and duct 306.In other embodiments, the passive tip gap control system may extendblade tip extensions 314 as the collective pitch of proprotor blades 304decreases into the VTOL flight mode to improve the performance of duct306. Even though the conversion from the forward flight mode to the VTOLflight mode of ducted aircraft 300 has been described as progressingwith collective pitch up rotation of proprotor systems 302, in otherimplementations, all proprotor systems 302 need not be operated at thesame time or at the same rate. Once ducted aircraft 300 has completedthe transition to the VTOL flight mode, ducted aircraft 300 may commenceits vertical descent to a surface. As best seen in FIG. 11H, ductedaircraft 300 has landed at the destination location.

Referring to FIGS. 12A-12F in the drawings, a passive tip gap controlsystem for proprotor system 332 including proprotor blades 334surrounded by duct 336 is schematically illustrated. Proprotor blades334 change collective pitch about pitch change axis 338. FIGS. 12A-12Bshow proprotor blades 334 having a higher collective pitch in theforward flight mode while FIGS. 12C-12D show proprotor blades 334 havinga lower collective pitch in the VTOL flight mode. Proprotor blades 334are retained on proprotor hub 340 by tension-torsion straps 342.Tension-torsion straps 342 may be partially or fully disposed inside ofproprotor blades 334. Tension-torsion straps 342 may extend fromproprotor hub 340 for any spanwise length along proprotor blades 334such as 10 percent, 20 percent, 30 percent, 40 percent or higher of thespanwise length of proprotor blades 334. In some embodiments,tension-torsion straps 342 may also be used to couple a proprotor blade334 to other proprotor blades 334. In addition to reacting centrifugalforce during the operation of proprotor system 332, tension-torsionstraps 342 twist and untwist to extend proprotor blades 334 at thehigher collective pitch in the forward flight mode as shown in FIGS.12A-12B and retract proprotor blades 334 at the lower collective pitchin the VTOL flight mode as shown in FIGS. 12C-12D. Each tension-torsionstrap 342 changes between the wound position shown in FIG. 12E and theunwound position shown in FIG. 12F based on the collective pitch ofproprotor blades 334. The lengths l₁ of tension-torsion straps 342 areless in the wound position than the lengths L₁ of tension-torsion straps342 in the unwound position. Thus, proprotor blades 334 are in theretracted position when tension-torsion straps 342 are in the woundposition as shown in FIG. 12C and in the extended position whentension-torsion straps 342 are in the unwound position as shown in FIG.12A.

Because proprotor system 332 has a slower rotational speed in theforward flight mode shown in FIG. 12A, unwinding tension-torsion straps342 at the higher collective pitch extends proprotor blades 334 tocompensate for shorter proprotor blades 334 at lower RPMs, therebymaintaining an approximately constant tip gap 344 in both flight modes.In other embodiments, tension-torsion straps 342 may be wound at thehigher collective pitch in the forward flight mode and unwound at thelower collective pitch in the VTOL flight mode to further retractproprotor blades 334 in the forward flight mode to provide an additionalsafety buffer between proprotor blades 334 and duct 336. The spanwisedistance that tension-torsion straps 342 extend and retract proprotorblades 334 may depend on various properties of tension-torsion straps342 such as distance 346 between bands 348, 350, the thickness of bands348, 350, the material from which tension-torsion straps 342 are formedand the overall length L₁ of tension-torsion straps 342. Thesecharacteristics of tension-torsion straps 342 may be varied to achievethe desired behavior of tip gap 344 in the various flight modes of theducted aircraft. In alternative embodiments, tension-torsion straps 342may instead be helical threaded screws or hydraulic pistons thatradially extend and retract proprotor blades 334. In yet otherembodiments, the passive tip gap control system described herein mayextend and retract proprotor blades 334 based on the cyclic pitch ofproprotor blades 334.

Referring to FIGS. 13A-13E in the drawings, a ducted aircraft 400including proprotor system 402 having proprotor blades 404 surrounded byduct 406 is schematically illustrated. Proprotor system 402 may be oneof a plurality of proprotor systems of ducted aircraft 400 that movesbetween vertical and horizontal orientations in forward flight and VTOLflight modes similar to ducted aircraft 10 described in FIGS. 1A-1F. Forflight conditions such as flight maneuvers or flight modes in which duct406 deforms due to flight loads, proprotor blades 404 could potentiallycome in contact with duct 406 as shown in FIGS. 13B-13C, potentiallyleading to a critical safety issue such as the failure of proprotorblades 404 or duct 406. To remedy this issue, proprotor blades 404include sacrificial blade tips 408 to allow the tips of proprotor blades404 to act as sacrificial portions of proprotor blades 404 to save duct406 and/or main bodies 410 of proprotor blades 404 from damage orfailure. The root ends of main bodies 410 of proprotor blades 404 arecoupled to proprotor hub 412 and sacrificial blade tips 408 are coupledto the distal ends of main bodies 410 of proprotor blades 404.

Sacrificial blade tips 408 each include a deformable core material 414and a shell layer 416 covering deformable core material 414. Shell layer416 has an airfoil cross-sectional shape and a closed distal end 418. Inother embodiments, distal end 418 of shell layer 416 may be an opendistal end to expose deformable core material 414. Shell layer 416provides wear resistance during high speed rotation of proprotor blades404 and general stiffness during flight, but is soft enough to deform orbreak away in the event of an impact between proprotor blades 404 andduct 406. Non-limiting examples of materials from which shell layer 416may be formed include fiberglass, fiberglass with spanwise alignedstrands, crumble-prone ceramic material, carbon-based material, sheetmetal, composite material, thin material, carbon fiber reinforcedplastic or frangible ceramic material. Deformable core material 414 isformed from a softer material than shell layer 416. For example,deformable core material 414 may be formed from foam. Shell layer 416abuts, overlaps or is adjacent to skin 420 of main body 410 as best seenin FIG. 13C. Sacrificial blade tip 408 is coupled to main body 410 usingadhesive 422 such as glue on a backing plate as well as adhesive tape424 at the outer interface boundary between shell layer 416 and skin 420of main body 410. Sacrificial blade tip 408 may be coupled to thebacking plate using hooks, fasteners, tabs, adhesive or other couplingtechniques.

FIGS. 13B-13C show the moment of contact between proprotor blades 404and duct 406. Such contact may occur, for example, upon deformation ofproprotor blades 404 and/or duct 406. The structure and materialcomposition of sacrificial blade tips 408 allow sacrificial blade tips408 to crumble, compress, crush or otherwise deform upon contact withduct 406. The aftermath of contact between sacrificial blade tips 408and duct 406 is shown in FIGS. 13D-13E, in which main bodies 410 ofproprotor blades 404 remain undeformed upon contact between sacrificialblade tips 408 and duct 406. The tip gap between proprotor blades 404and duct 406 is also increased upon the deformation of sacrificial bladetips 408 to allow the uninterrupted functioning of ducted aircraft 400for continued operation or an emergency landing. Sacrificial blade tips408 may be designed to be inexpensive and lightweight, and may beeconomically replaced when ducted aircraft 400 returns from a mission.Sacrificial blade tips 408 may be designed for one-time use in that theyare replaced, as opposed to repaired, when they deform as a result ofcontact with duct 406.

Referring to FIGS. 14A-14D in the drawings, a proprotor system 430having proprotor blades 432 surrounded by duct 434 is schematicallyillustrated. Proprotor blade 432 includes sacrificial blade tip 436coupled to the distal end of main body 438. Sacrificial blade tip 436includes deformable core material 440 covered by shell layer 442.Sacrificial blade tip 436 is coupled to main body 438 of proprotor blade432 at an interface boundary 444. Interface boundary 444 is illustratedas having a rounded distal end but may be squared off in otherembodiments as shown in FIGS. 13C and 13D. In the illustratedembodiment, shell layer 442 is integral with skin 446 of main body 438to form an integral layer covering interface boundary 444. Shell layer442 also has a closed distal end. Deformable core material 440 forms alattice structure made from polymer or another frangible material. Inthe illustrated embodiment, the lattice structure formed by deformablecore material 440 is a honeycomb structure, although other latticeinfill shapes such as triangular, grid or irregular lattices may also beformed by deformable core material 440. When sacrificial blade tip 436contacts duct 434 as shown in FIGS. 14A-14B, a fragment 448 ofsacrificial blade tip 436 breaks away, leaving a slightly shorterproprotor blade. In other embodiments, sacrificial blade tip 436 mayseparate from main body 438 at interface boundary 444 upon contact withduct 434. The breaking away of fragment 448 enlarges the tip gap betweenproprotor blade 432 and duct 434 to prevent further damage to thesecomponents.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A tip gap monitoring and control system for aducted aircraft having a vertical takeoff and landing flight mode and aforward flight mode, the tip gap monitoring and control systemcomprising: a flight control computer comprising: a tip gap monitoringsystem including one or more sensors; and a blade length control moduleconfigured to generate a blade tip actuator command in response to theaircraft converting between the vertical takeoff and landing flight modeand the forward flight mode; and a proprotor system in datacommunication with the flight control computer, the proprotor systemcomprising: a duct; a plurality of proprotor blades surrounded by theduct, each of the proprotor blades including an active blade tip movableinto a plurality of positions including a retracted position and anextended position; and one or more actuators coupled to the active bladetips; wherein, the tip gap monitoring system is configured to monitor atip gap between the proprotor blades and the duct; and wherein, the oneor more actuators are configured to move the active blade tips betweenthe retracted position and the extended position based on the blade tipactuator command, thereby controlling the tip gap between the proprotorblades and the duct.
 2. The tip gap monitoring and control system asrecited in claim 1 wherein the blade tip actuator command furthercomprises a tip gap adjustment distance, the one or more actuatorsconfigured to move the active blade tips by the tip gap adjustmentdistance.
 3. The tip gap monitoring and control system as recited inclaim 1 wherein the blade tip actuator command further comprises one ormore blade-specific blade tip actuator commands, each of the one or moreblade-specific blade tip actuator commands corresponding to a respectiveone of the active blade tips.
 4. The tip gap monitoring and controlsystem as recited in claim 1 wherein each of the proprotor bladesfurther comprises a main body having an open distal end, the activeblade tips slidably coupled to the open distal ends of the main bodiesof the proprotor blades; and wherein, the active blade tips are at leastpartially retracted into the main bodies of the proprotor blades in theretracted position and at least partially extended from the open distalends of the main bodies of the proprotor blades in the extendedposition, thereby increasing the tip gap in the retracted position. 5.The tip gap monitoring and control system as recited in claim 1 whereineach of the proprotor blades further comprise a main body having adistal end, the active blade tips hingeably coupled to the distal endsof the main bodies of the proprotor blades; and wherein, the activeblade tips are substantially coplanar with the main bodies of theproprotor blades in the extended position and form an angle of less than180 degrees with the main bodies of the proprotor blades in theretracted position, thereby increasing the tip gap in the retractedposition.
 6. The tip gap monitoring and control system as recited inclaim 1 wherein each proprotor blade further comprises a spring biasinga respective active blade tip in one of the retracted position or theextended position.
 7. The tip gap monitoring and control system asrecited in claim 1 wherein the active blade tips are independentlyactuated to permit nonuniform positioning of the active blade tips. 8.The tip gap monitoring and control system as recited in claim 1 whereinthe proprotor system further comprises a proprotor hub, the one or moreactuators disposed at the proprotor hub; and wherein, the one or moreactuators are coupled to the active blade tips by a plurality ofspanwise links.
 9. The tip gap monitoring and control system as recitedin claim 1 wherein the one or more actuators further comprise aplurality of actuators disposed at distal ends of the proprotor blades,each actuator coupled to a respective one of the active blade tips. 10.The tip gap monitoring and control system as recited in claim 1 whereinthe one or more sensors are coupled to the proprotor system; andwherein, the one or more sensors include at least one of a strain gauge,an accelerometer or a distance sensor.
 11. A rotorcraft having avertical takeoff and landing flight mode and a forward flight mode, therotorcraft comprising: a fuselage; a flight control computer comprising:a tip gap monitoring system including one or more sensors; and a bladelength control module configured to generate a blade tip actuatorcommand in response to the rotorcraft converting between the verticaltakeoff and landing flight mode and the forward flight mode; and aproprotor system coupled to the fuselage and in data communication withthe flight control computer, the proprotor system comprising: a duct; aplurality of proprotor blades surrounded by the duct, each of theproprotor blades including an active blade tip movable into a pluralityof positions including a retracted position and an extended position;and one or more actuators coupled to the active blade tips; wherein, thetip gap monitoring system is configured to monitor a tip gap between theproprotor blades and the duct; and wherein, the one or more actuatorsare configured to move the active blade tips between the retractedposition and the extended position based on the blade tip actuatorcommand, thereby controlling the tip gap between the proprotor bladesand the duct.
 12. The rotorcraft as recited in claim 11 furthercomprising a maneuver detection module configured to detect a flightcondition of the rotorcraft, the blade length control module configuredto determine the blade tip actuator command based on the flightcondition; wherein, the flight condition further comprises at least oneof a flight maneuver or a flight mode.
 13. A method for controlling atip gap for a ducted aircraft having a vertical takeoff and landingflight mode and a forward flight mode, the method comprising: monitoringa tip gap between a duct and a plurality of proprotor blades using oneor more sensors; generating a blade tip actuator command in response tothe ducted aircraft converting between the vertical takeoff and landingflight mode and the forward flight mode; transmitting the blade tipactuator command to a proprotor system including the duct and theproprotor blades, the proprotor blades including active blade tips; andmoving at least one of the active blade tips between a retractedposition and an extended position in response to the blade tip actuatorcommand, thereby controlling the tip gap between the proprotor bladesand the duct.
 14. The method as recited in claim 13 further comprisinggenerating the blade tip actuator command in response to receiving a tipgap adjustment distance.
 15. The method as recited in claim 14 furthercomprising generating the blade tip actuator command in response to thetip gap adjustment distance exceeding a tip gap adjustment distancethreshold.
 16. The method as recited in claim 13 further comprisinggenerating the blade tip actuator command based on a pitch of theproprotor blades.
 17. The method as recited in claim 13 furthercomprising retracting the active blade tips in response to detecting astructural deformity of the proprotor system.
 18. The method as recitedin claim 13 further comprising retracting the active blade tips inresponse to detecting a collision with the proprotor system.
 19. Themethod as recited in claim 13 further comprising moving the active bladetips such that the tip gap is substantially equal to a tip gap target.20. The method as recited in claim 13 further comprising: moving atleast one of the active blade tips to form a vertical takeoff andlanding flight mode tip gap between the proprotor blades and the duct inresponse to the ducted aircraft converting to the vertical takeoff andlanding flight mode; and moving at least one of the active blade tips toform a forward flight mode tip gap between the proprotor blades and theduct in response to the ducted aircraft converting to the forward flightmode, the forward flight mode tip gap different from the verticaltakeoff and landing flight mode tip gap.